Honey Bee-Infecting Plant Virus with Implications on Honey Bee
Colony Health
Michelle L. Flenniken
Department of Plant Sciences and Plant Pathology, Montana State University, Bozeman, Montana, USA
ABSTRACT Honey bees are eusocial insects that are commercially managed to provide pollination services for agricultural crops.
Recent increased losses of honey bee colonies (averaging 32% annually since 2006) are associated with the incidence and abun-
dance of pathogens. In their study inmBio, J. L. Li et al. [mBio 5(1):e00898-13, 2014, doi:10.1128/mBio.00898-13] share their dis-
covery that a plant virus, tobacco ring spot virus (TRSV), replicates in honey bees and that the prevalence of this virus was high
in weak colonies. Their findings increase our understanding of the role of viruses in honey bee colony losses and underscore the
importance of surveying for new and/or emerging viruses in honey bees. Furthermore, their findings will pique the interest of
virologists and biologists across all disciplines. The discovery that a plant virus (TRSV) replicates, spreads, and negatively affects
the health of an insect host will lead to additional studies on the mechanisms of host-specific adaptation and the role of cross-
kingdom infections in the transmission of this virus.
Honey bees (Apis mellifera) pollinate numerous agriculturalcrops, including almonds, apples, blueberries, and oil seed
crops, valued at a total of $14.6 billion annually (1). They are also
responsible for pollination of plant species that augment the bio-
diversity of agricultural and nonagricultural landscapes. Increased
annual losses of honey bee colonies in the United States, Canada,
and Europe have alarmed agricultural specialists, beekeepers,
growers, scientists, and the general public. These losses are in part
due to a phenomenon known as colony collapse disorder (CCD),
which occurs predominantly over the fall/winter months. The
cause(s) of increased colony losses remain unknown, but the cur-
rent prevailing theory is that multiple factors (i.e., pathogens,
poor bee nutrition, and agrochemical exposure) acting in concert
can reduce colony longevity (2, 3). While no specific pathogen(s)
has been shown to be consistently associated with CCD and/or
overwinter losses, pathogen incidence and abundance correlate
with colony loss (2, 4). Therefore, continued efforts to monitor
and discover bee pathogens are extremely important to agricul-
ture and global food production. Furthermore, these research ef-
forts lead to extremely interesting biological findings, including
those of Li et al., as reported recently in this journal (5).
Li et al. discovered that a plant virus, tobacco ring spot virus
(TRSV), infects and replicates in honey bees. This is a fascinating
example of an RNA virus that can infect both insect and plant
hosts. Furthermore, Li et al. determined that weak honey bee col-
onies, defined by low colony population size and collapse, had a
higher prevalence of TRSV than healthy colonies (5). These find-
ings expand the realm of pathogens that may play important roles
in honey bee health.
The majority of honey bee viruses are positive-sense, single-
stranded RNA (ssRNA) viruses in the order Picornavirales (re-
viewed in reference 6). Taxonomic classification, as inferred from
phylogenetic analyses of virus-encoded RNA-dependent RNA
polymerases (RdRp), further delineates picorna-like viruses into
the Picornaviridae, Dicistroviridae, and Iflaviridae families (7).
Common honey bee viruses in the Dicistroviridae family include
acute bee paralysis virus (ABPV), black queen cell virus (BQCV),
Israeli acute paralysis virus (IAPV), andKashmir bee virus (KBV).
Iflaviruses include Sacbrood virus (SBV), deformed wing virus
(DWV), and slow bee paralysis virus (SBPV). Additional viruses,
including chronic bee paralysis virus (CBPV), Kakugo virus, and
the Lake Sinai viruses (LSVs), remain unclassified (8–10). Virus
infections in honey bees can be asymptomatic, cause deformities
and/or paralysis, or result in death.
Advances in molecular techniques, including genome se-
quencing, PCR-mediated detection, microarray-based detection
and discovery, and ultra-high-throughput (or next-generation)
sequencing have increased our understanding of virus prevalence,
phylogenetic relatedness, and association with colony health (4, 6,
10–14). Recently, next-generation sequencing of honey bee-
associated viruses determined the prevalence of Israeli acute pa-
ralysis virus in samples from CCD-affected bees (12), identified a
new honey bee-associated strain of aphid lethal paralysis virus
(ALPV; strain Brookings) (10, 15), and resulted in the discovery of
a new group of honey bee-infecting viruses, the LSVs (10), which
have been associated with CCD in the United States (4) and over-
winter losses in Belgium (16). These recent discoveries, together
with those of Li et al., underscore the importance of research
aimed at discovering, detecting, and characterizing the pathogen-
esis, phylogenetic relatedness, and geographic distribution of
honey bee viruses, as well as the investigation of these pathogens in
the context of the entire honey bee microbiome (17–19).
Like the majority of honey bee viruses, TRSV is a positive-
sense ssRNA virus within the Picornavirales order. TRSV was first
observed in infected tobacco and is known to infect numerous
plant species, including crops, weeds, and ornamentals. TRSV is
vectored by nematodes and insects (e.g., aphids, thrips, grasshop-
pers, and the tobacco flea beetle). Thework by Li et al. significantly
expands the knownhost range of TRSV to include honey bees. The
phylogenetic relationship between common honey bee viruses
and TRSV inferred from the RdRp sequence is well illustrated in
the work of Koonin et al. (7), which includes honey bee viruses on
Published 25 February 2014
Citation Flenniken ML. 2014. Honey bee-infecting plant virus with implications on
honey bee colony health. mBio 5(2):e00877-14. doi:10.1128/mBio.00877-14.
Copyright © 2014 Flenniken. This is an open-access article distributed under the terms
of the Creative Commons Attribution-Noncommercial-ShareAlike 3.0 Unported license,
which permits unrestricted noncommercial use, distribution, and reproduction in any
medium, provided the original author and source are credited.
Address correspondence to Michelle L. Flenniken, michelle.flenniken@montana.edu.
COMMENTARY
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two branches and TRSV on the third branch of a large virus clade
composed of chromalveolate-, plant-, and arthropod-infecting vi-
ruses.
Interestingly, the majority of viruses transmitted by insects to
animal hosts (i.e., arboviruses) replicate within insect vectors,
whereas the majority of insect-transmitted plant viruses do not
replicate in insect vectors (20). Therefore, the finding of Li et al.
that TRSV replicates in honey bees is particularly fascinating. Fur-
thermore, the distribution of TRSV in honey bees differs from that
seen in insects that serve as vectors primarily for plant viruses.
Specifically, TRSV was detected throughout the honey bee body,
whereas plant viruses that replicate in aphid and thrip vectors are
found primarily in the gut and salivary gland. Li et al. demon-
strated that the Varroa destructor mite, an ectoparasite of honey
bees that transmits some honey bee viruses, also harbors TRSV.
The distribution pattern of TRSV in theVarroa destructormite, as
demonstrated by in situ hybridization, is similar to that of arthro-
pod viral vectors. Future studies that determine whether TRSV
replicates in Varroa, and whether it indeed transmits TSRV to
honey bees, are important for understanding the transmission dy-
namics of this newly described honey bee pathogen.
The phylogenetic relationship among the honey bee, Varroa
mite, and pollen-associated TRSV isolates described by Li et al., as
well as plant TRSV isolates in the NCBI database, was inferred
from a 731-nucleotide, capsid protein-encoding region of the ge-
nome. The resulting phylogenetic tree indicated that the bee,mite,
and pollen isolates are indistinguishable from each other but dis-
tinct from the plant isolates. Full-length genome sequences of
TRSV isolates are needed to address several questions, including
the following. Are there host-specific TRSV mutations, particu-
larly in viral proteins that interact with the host (e.g., capsid pro-
teins)? Do TRSVs isolated from plants have increased conserva-
tion in the genome region encoding the movement protein (MP),
which facilitates virus passage through plant plasmodesmata?Will
the TSRV VPg (virus protein, genome-linked) genome region in
plant and bee isolates diverge over time? Does this virus regularly
shuttle between plant and insect hosts, and if so, will host-specific
mutations be difficult to confirm? Comparative analyses of TRSV
isolates from historic samples and currently circulating isolates
from diverse geographic regions may be used to begin to address
some of these exciting, biologically relevant questions. Likewise,
additional studies aimed at assessing the host range of TRSV in
other insects (e.g., bees, wasps, and ants) will further our under-
standing of the interspecies transmission.
Clearly the findings by Li et al. will excite scientists from all
fields, since the discovery that a plant virus (TRSV) infects and
replicates in honey bees is a unique finding with broad implica-
tions. The observation that TRSV infection correlates with poor
colony health and colony collapse warrants further investigation
and may result in a better understanding of increased honey bee
losses. This work is particularly relevant for researchers examining
correlations between individual bee and colony health with
pathogen- and host-associated microbial profiles. Poor colony
health and colony losses are associated with increased pathogen
incidence and abundance. Since the majority of honey bee patho-
gens are RNA viruses, it is likely that RNA interference (RNAi)-
mediated antiviral strategies are employed by honey bees to con-
trol viral invasion (21–23), much like fruit flies and mosquitos
(reviewed in references 24 and 25). However, basic questions re-
garding the mechanism(s) of honey bee antiviral immunity re-
main unanswered. For example, our studies demonstrate that
double-stranded RNA (dsRNA) of any specificity decreases virus
load in honey bees, which indicates the potential of additional
dsRNA-triggered immunemechanisms (26); likewise, other stud-
ies demonstrate unexpected off-target effects of dsRNA (27). To-
gether, these studies suggest that antiviral defense in honey bees
involves RNAi and additional dsRNA-triggered antiviral immune
pathways. Future studies are needed to determine the molecular
mechanisms and relative importance of these responses in specific
bee-virus interactions.
As Li et al. have demonstrated, basic science often leads to the
discovery of the unexpected. We anticipate that future experi-
ments in this field will lead to the discovery of additional bee
pathogens and reveal the mechanisms of honey bee antiviral im-
munity. These studies will result in a greater understanding of the
impact of viruses on honey bee health. Such discoveries may re-
define our understanding of honey bee health, immunity, and
disease transmission and are paramount to understanding the
current crisis facing honey bees.
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